A spokesperson for the Japanese government says authorities are presuming that "possible meltdowns are under way at two nuclear reactors" at this hour.

Chief Cabinet Secretary Yukio Edano told reporters there is a "possibility" of a meltdown at the plant's No. 1 reactor, adding, "It is inside the reactor. We can't see." He then added that authorities are also "assuming the possibility of a meltdown" at the facility's No. 3 reactor.A third reactor at the Fukushima #1 power plant in Japan has lost its emergency cooling functionality. Now, six total reactors have failed at two Fukushima nuclear power plants since the earthquake and tsunami hit.
BBC News overview here.

Mainichi News reports that at least 15 people have been hospitalized for radiation exposure. At the time of this blog post, NHK television is reporting a slightly higher count: 133 evacuees have been tested for radiation exposure, 19 of them have confirmed contaminated. First responders are also distributing Iodine tablets to potentially affected populations.

Japan's Kyodo news agency reports that radiation levels have risen above the legal safety limit around the No. 1 Fukushima plant, and the matter is now considered an "emergency situation."

Asahi Shimbun reports that Japan's central government has declared a state of emergency at the No. 2 Fukushima nuclear power plant, which sits about 8 km south of the No. 1 Fukushima plant where the previous explosion occurred.

A state of emergency had been declared for the No. 1 plant the previous day. Residents within 10 kilometers of the No. 1 plant were told to evacuate. Traces of radioactive cesium were confirmed around the plant in an inspection. But how much? And how far out does the risk of radioactive contamination reach?

William Broad writes in the New York Times:

The different radioactive materials being reported at the nuclear accidents in Japan range from relatively benign to extremely worrisome. The central problem in assessing the degree of danger is that the amounts of various radioactive releases into the environment are now unknown, as are the winds and other atmospheric factors that determine how radioactivity will disperse around the stricken plants.But the situation is not good, and as more data becomes available, the news isn't improving.

Here's an updated NYT overview on efforts to prevent a greater crisis at the Fukushima plants, now that a second reactor has failed.

In just the past few hours, Tokyo Electric Power Company (TEPCO, the company that operates the nuclear plants in question) has started releasing air from a reactor container vessel at the plant's No.3 reactor.

In this AP article, some possible next-step scenarios if these current efforts to cool down the reactors fail:

If the reactor core became exposed to the external environment, officials would likely began pouring cement and sand over the entire facility, as was done at the 1986 Chernobyl nuclear accident in the Ukraine, Peter Bradford, a former commissioner of the U.S. Nuclear Regulatory Commission, said in a briefing for reporters.
At that point, Bradford added, "many first responders would die."
Don't miss Boing Boing science editor Maggie Koerth-Baker's "explainer" on how nuclear power plants work: "Nuclear energy 101: Inside the "black box" of power plants".

holy jeez japan... is going to be like one big Chernobyl. Im pretty sure that in 86 only 1 or 2 went, but six? Even with 1 or 2 reactors look at the vast area it coveres in Chernobyl... and compare that to Japan. Why not china ?

I mean... well actually maybe its karma for those cats they put in the bottles and unit 731.

holy jeez japan... is going to be like one big Chernobyl. Im pretty sure that in 86 only 1 or 2 went, but six? Even with 1 or 2 reactors look at the vast area it coveres in Chernobyl... and compare that to Japan. Why not china ?

I mean... well actually maybe its karma for those cats they put in the bottles and unit 731.

It could be worse. I think it will. Russia responded to chernobal very quickly. Japan is in denial